Add helpers to provide CPUID-based guest vendor checks, i.e. to do theugly register comparisons. Use the new helpers to check for an AMDguest vendor in guest_cpuid_is_amd() as well as in the existing emulatorflows.

Using the new helpers fixes a _very_ theoretical bug whereguest_cpuid_is_amd() would get a false positive on a non-AMD virtual CPUwith a vendor string beginning with "Auth" due to the previous logiconly checking EBX. It also fixes a marginally less theoretically bugwhere guest_cpuid_is_amd() would incorrectly return false for a guestCPU with "AMDisbetter!" as its vendor string.